1. Field of the Invention
The present invention relates to an optical fiber and an optical fiber core for which the optical fiber is utilized. More particularly, the present invention relates to a resin coating around a bare optical fiber that composes the optical fiber.
2. Description of the Related Art
In recent years, optical fibers have become popular to use, and thus optical fiber cables have been laid down in a variety of environments. In some environments, since water may infiltrate into the inside of optical fiber cables, optical fibers are required to be highly reliable for a long period of time.
Optical fibers made of fused silica are used for optical communication. Generally, such an optical fiber has a structure in which a first layer (primary layer) with a relatively soft resin and a second layer (secondary layer) with a relatively hard resin are laminated around a bare optical fiber (glass optical fiber) that is made of fused silica, and is composed of a core and a clad. The first layer (primary layer) contributes to reducing lateral pressure to the bare optical fiber, and the second layer (secondary layer) contributes to preventing damage to the bare optical fiber. Ultraviolet-curable resins are mostly used as the resins described above.
When conventional optical fiber cores are exposed to water derived from environments, close contact between primary layers and bare optical fibers will be changed. For example, a decrease of the close contact may cause projection, microbend or the like of bare optical fibers, and thereby transmission loss of the optical fiber cores may be increased. Also, when the close contact is increased, sometimes primary layers covering bare optical fibers become difficult to remove. Note that such removal is required for maintenance and the like for optical fiber cores.
To solve the problem, for example, Japanese Unexamined Patent Application, First Publication No. 2007-334111 discloses a method for suppressing detachment of an interface between a glass optical fiber (bare optical fiber) and a flexible covering layer, by defining a relationship between Young's modulus of the flexible covering layer (primary layer) and Young's modulus of a rigid covering layer.
In conventional optical fiber cores, close contact between bare optical fibers and primary layers is not sufficiently stable, and therefore increased stability has been desired.
The present invention has been completed in light of the above situation, and an object of the present invention is to provide an optical fiber core, in which close contact between a bare optical fiber and a primary layer is stably maintained even if the optical fiber core is used under an environment in which it has been exposed to water.